For some time now, web workers with more than one Google account, or a personal Google/Google Apps…
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To activate the feature, head over to your accounts page and select "multiple log-in." It's in the process of rolling out, so if you don't see it now just hold tight.

Clicking on "Change" enables a drop-down option next to your email email address on the top right of the page, allowing you to switch back and forth with quickness and ease. Simple as that.

Now the caveats: multiple log-in doesn't work for mobile, the Calendar gadget is apparently wonky in Gmail, and the service only works for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. That's a still a wide range of services, and more than enough functionality for such a highly anticipated feature. [Google Operating System]